There is a growing need for air separation processes which produce nitrogen and oxygen at medium pressures, i.e. pressures between 10 and 75 psig. For example, the float glass industry presently requires the use of nitrogen in its furnace as an inerting atmosphere. This nitrogen typically must be supplied from the air separation unit at 25 psig. There is also an emerging use of oxygen in float gas facilities for enrichment of air to the burners or for oxy-fuel burners. This oxygen does not have to be high purity but is required at pressures of about 25 psig. A typical float glass plant will require about 150,000 SCFH of nitrogen and 50,000 SCFH of 95% oxygen. The aluminum manufacturing industry also has similar nitrogen requirements and the potential for the need of medium pressure oxygen in its burners. This requirement of providing both nitrogen and oxygen at medium pressure raises a problem for the industrial gas industry. That problem being what is the most economical method of supplying these oxygen/nitrogen requirements.
The conventional method of supplying oxygen and nitrogen at these medium pressures has been to use a low pressure air separation unit in which the low pressure column works at 2-9 psig. The oxygen and nitrogen products are then compressed to required pressure.
U.S. Pat. Nos. 2,918,802 and 3,086,371 disclose pumped liquid oxygen processes in which liquid oxygen is vaporized and warmed against a part of the air feed. This eliminates the requirement for an oxygen compressor but adds a LOX pump to the process and complicates the air compression by requiring two air compressors.
Patent application U.S. Ser. No. 07/564,803 discloses another pumped liquid oxygen process.
U.S. Pat. No. 4,617,036 discloses a process in which the low pressure column is operated at about 45 to 70 psia. The nitrogen product from the column is warmed against the air. This process produces the nitrogen product at the desired pressure without a need for additional pressure, but, unfortunately, medium pressure oxygen is not produced.